fixed comment retrieval

lms/templates/discussion/_ajax_single_thread.html

-<%namespace name="renderer" file="_thread.html"/>
-${renderer.render_comments(thread['children'])}
+<%namespace name="renderer" file="_content_renderer.html"/>
+${renderer.render_comments(thread.get('children'))}

lms/templates/discussion/_content_renderer.html

@@ -7,7 +7,9 @@
 <%def name="render_content_with_comments(content)">
   <div class="${content['type']}" _id="${content['id']}" _discussion_id="${content.get('commentable_id')}" _author_id="${helpers.show_if(content['user_id'], content.get('anonymous'))}">
     ${render_content(content)}
-    ${render_comments(content.get('children', []))}
+    % if content.get('children') is not None:
+      ${render_comments(content['children'])}
+    % endif
   </div>
 </%def>
 

lms/templates/discussion/_single_thread.html

-<%namespace name="renderer" file="_thread.html"/>
+<%namespace name="renderer" file="_content_renderer.html"/>
 
 <section class="discussion" _id="${discussion_id}">
   <a class="discussion-title" href="javascript:void(0)">Discussion</a>
-  ${renderer.render_thread(course_id, thread, show_comments=True)}
+  ${renderer.render_content_with_comments(thread)}
 </section>
 
 <%include file="_js_data.html" />